Diapositiva 1

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GALAXIAS
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Optico
Infrarojo
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The Galactic Disk
The disk of our Galaxy is made up of three main
components

• Stars
• Gas
• Dust

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The gas in between stars is the Interstellar
Medium (or ISM)
Like everywhere, most gas in the disk is hydrogen.
Molecular (H2) Cold, dense, tightly
clumped. Stars form within molecular
clouds. Neutral or Atomic (HI) Cool, less
dense, less tightly clumped. Most common phase
the reservoir for forming the next generation of
stars. Ionized (HII) Hot, more
diffuse. By-product of forming young stars.
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Young massive stars die out, and electrons and
nuclei recombine
Gas is compressed, and cools
Atomic HI
Molecular H2
Ionized HII
Young massive O-stars form, and ionize the gas
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Fountains and Blowouts
SNe from newly formed massive stars can make
holes in the disk, driving material out of the
Galaxy or redistributing it
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Blowout in M82 starburst galaxy
Red is false color, showing location of
extremely hot gas detected in X-rays.
Hot because the gas is mostly the ejecta from
supernovae!
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Los brazos de la galaxia son regiones de alta
densidad donde ocurre la mayor parte del proceso
de formación de nuevas estrellas.
Por otro lado, las ondas de choque generadas por
explosiones de estrellas evitan que los brazos se
dispersen.
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Los brazos no son rígidos más bien son ciertas
zonas que las estrellas atraviesan en su
movimiento de rotación alrededor del centro
galáctico.
Se calcula que el Sol ya dio varias vueltas
completas. En este momento, está en una zona
"tranquila", al borde de un brazo secundario.
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The Bulge A Very Crowded Neighborhood
The density of stars in the bulge is about
50,000 per cubic parsec By comparison, the
nearest star to the Sun is 1.3 pc away!
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Milky Way is believed to have a bar at its center
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The Galactic Halo Ancient Stars
No gas, so no star formation just a bunch of
old stars, either by themselves or in globular
clusters.
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Stuff in Galaxies moves in two basic ways
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Side View of the Milky Way
The halo is really the stellar halo turns
out theres actually a larger halo we cant even
see!
Globular clusters live in the halo
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Turns out theres a lot of this invisible matter.
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Overall Properties of the Galactic Disk, Halo,
and Bulge
Property Disk Halo Bulge
Shape highly flattened like a frisbee roughly spherical mildly flattened MM shaped
Star Types young and old only old stars young and old stars more old stars at greater distances from center
ISM gas and dust no gas or dust gas and dust, especially in inner regions
New Stars? ongoing star formation no star formation for past 10 billion years ongoing star formation in inner regions
Dynamics contents move in circular orbits in the Galactic plane stars have random orbits in three dimensions largely random orbits with some net rotation about the center
Substructure spiral arms no obvious substructure nucleus ring of gas and dust near center bar
Color overall white color with blue spiral arms stars reddish in color due to old age and cool temperatures yellow-white due to mix of stars
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Observational tracers of magnetic fields

• Polarization of starlight perpendicular field
  in 2 or 3 kpc
• orientation // B? -------------
  9000 stars
• Zeeman splitting parallel field, in situ
  (masers, clouds)
• ?? ? B// ------ 30
  masers
• Polarization at infrared, mm perpendicular
  field orientation // B? ------ clouds star
  formation regions
• Synchrotron radiation vertical field
  structures (added)
• total intensity S ? B?2/7, p ?
  B?u2 / B?t2
• Faraday rotation parallel field, integrated
  (the halo disk)
• RM ? ? ne B// ds ------ 500 pulsars
  gt1000 EGSes

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Large-scale magnetic field in the Galactic disk
The largest coherent field structrue detected in
the Universe!
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Poloidal Toroidal fields near GC

• Toroidal fields
• (Novak et al. 2003, 2000)
• permeated in the
• central molecular zone
• (400pc50pc)
• sub-mm obs of p
• toroidal field directions
• determined by averaged
• RMs of plumes or SNR!
• Poloidal field
• filaments Unique to GC
• --- dipolar geometry!
• (Morris 1994 Lang et al.1999)

(from Novak et al. 2003)
Predicted B-direction
GC
150pc
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The Milky Way A Barred Spiral Galaxy
The Milky Way is a spiral galaxy, sometimes
also called a late type galaxy.
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Side View edge-on
Top View face-on
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Sombrero Galaxy
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• Elliptical Galaxies (or just ellipticals)
• No disk! old!

spheroidals
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S0 galaxies Like ellipticals, but usually a
bit flatter.
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Early Types
Late Types
Unbarred and Barred Spirals
Ellipticals
Lenticulars
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Varying amounts of bulge disk components
suggests different formation evolution history
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Early-Type Galaxies from the Sloan Digital Sky
Survey (SDSS)
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Late-Type Galaxies From SDSS
(red because of dust)
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There are galaxies beyond the Hubble Sequence
that continue this trend.
???
Dwarf or Irregular Galaxies
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Dwarf or Irregular galaxies tend to have more
chaotic appearances

• Low mass (107-109 stars, vs 1010 for spirals)
• High star formation rates (usually)
• No obvious bulge or spiral patterns.
• Most numerous type of galaxy in the Universe!

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Dwarf galaxies from the Sloan Digital Sky Survey.
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Gas Infall

• Galaxies continue to form stars.
• Just enough gas in galaxy disks today to form
  stars for lt109 years.
• Fresh gas must keep it going.
• Fraction of metals (non-H, He) in stars is lower
  than expected.
• Fresh Hydrogen must be flowing in.

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Merging or Galaxy Interactions

• Gravity pulls galaxies together!
• They can orbit each other eventually merge!

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The Antennae (Hubble Image _at_ Right)
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Multiple cores in some ellipticals in clusters of
galaxies
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Minor Mergers
We know this is currently happening
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Picture credit W. Keel
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Eagle Nebula (M16)
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Eagle Nebula (M16)
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Eagle Nebula (M16)
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Eagle Nebula (M16)